Direct Comparison of Leaf Plasmodesma Structure and Function in Relation to Phloem-Loading Type

Abstract

The export of photosynthetically produced sugars from leaves depends on plasmodesmatal transport of sugar molecules from mesophyll to phloem. Traditionally, the density of plasmodesmata (PD) along this phloem-loading pathway has been used as a defining feature of different phloem-loading types, with species proposed to have either many or few PD between the phloem and surrounding cells of the leaf. However, quantitative determination of PD density has rarely been performed. Moreover, the structure of PD has not been considered, even though it could impact permeability, and functional data are only available for very few species. Here, a comparison of PD density, structure, and function using data from transmission electron microscopy and live-cell microscopy was conducted for all relevant cell-cell interfaces in leaves of nine species. These species represent the three principal phloem-loading types currently discussed in literature. Results show that relative PD density among the different cell-cell interfaces in one species, but not absolute PD density, is indicative of phloem-loading type. PD density data of single interfaces, even combined with PD diameter and length data, did not correlate with the intercellular diffusion capacity measured by the fluorescence loss in photobleaching method. This means that PD substructure not visible on standard transmission electron micrographs may have a strong influence on permeability. Furthermore, the results support a proposed passive symplasmic loading mechanism in the tree species horse chestnut (Aesculus hippocastanum), white birch (Betula pubescens), orchard apple (Malus domestica), and gray poplar (Populus x canescens) as functional cell coupling and PD structure differed from active symplasmic and apoplasmic phloem-loading species.

Reticulon‐like properties of a plant virus‐encoded movement protein

“…The PD channels represent the main pathway for intercellular exchange of molecules (Bayer et al, 2014;Otero et al, 2016) and it has been known for many years that they are exploited by plant viruses for their cell-to-cell movement (Tomenius et al, 1987;Wolf et al, 1989;Heinlein, 2015b). The PD channel has a cytoplasmic space delimited by the plasma membrane (PM) and an axial appressed tubule of endoplasmic reticulum (ER) termed 'desmotubule', therefore looking like a system of two membrane tubes inserted one into the other (Bell & Oparka, 2011;Liesche et al, 2019). The PM and desmotubule membranes are located close to each other, at a distance of c. 10-15 nm, and the cytoplasmic, intermembrane space contains spoke-like, presumably proteinaceous, elements that emanate from the desmotubule and link the two membranes (Roberts & Oparka, 2003;Petit et al, 2020).…”

Penicillium chrysogenum polypeptide extract protects tobacco plants from tobacco mosaic virus infection through modulation of ABA biosynthesis and callose priming

“…The sections were then observed and imaged with a transmission electron microscope (TECNAI G2, FEI, USA). The plasmodesma diameters were measured at the neck region via the TEM Imaging & Analysis software (FEI, USA; Schindelin et al , 2012 ; Liesche et al , 2019 ).…”

Plasmodesmata Conductivity Regulation: A Mechanistic Model

“…The second way to change the intercellular conductivity is the modification of Pd structure when a transition from primary to secondary Pd occurs. The study of the sink-source transition in tobacco leaves has shown how this mechanism works [28] when a large number of simple primary Pd are lost and the contents of branched secondary Pd increase [26,40].…”

“…Pd provide a controlled symplastic exchange of both low molecular weight compounds and larger molecules, such as signaling molecules, transcription factors, non-cell-autonomous plant proteins [17,18], small regulatory RNAs and messenger RNAs [19,20], between plant cells, allowing general reactions in their growth processes and in response to stressful environmental factors [21][22][23]. Through Pd and phloem, photosynthesis products from the mesophyll of source leaves are exported to sink leaves and the plant growth point [24][25][26]. Sucrose, as a primary carbohydrate transported long distances in many plant species, is loaded into the phloem and unloaded into distal sink tissues.…”